1. Field of the Invention
The present invention relates to a system for transmitting and receiving train control signals using loop coils.
2. Description of the Related Art
FIG. 8 shows a state where four antennas A disposed on a train 1 at locations other than a cross point 3 are transmitting and receiving ground signals and on-vehicle signals when given ground signals are applied to a loop coil 2 from a trackside transceiver 4.
In the description, the cross point 3 refers to a crossing point of a loop coil disposed to solve the imbalance of inductance and capacitance of the loop coil.
Ground signals output from the trackside transceiver 4 are applied to the loop coil 2, and the loop coil 2 generates a magnetic field by the current components of the ground signals.
The antennas A disposed on the train 1 receive the magnetic field generated from the loop coil 2 via a magnetic coupling and transmit the ground signals to an on-vehicle transceiver 6 disposed on the train 1.
Further, the antennas A disposed on the train 1 generate a magnetic field by the current components of the on-vehicle signals output from the on-vehicle transceiver 6 and transmit the on-vehicle signals via the magnetic coupling to a trackside receiver 5.
The train 1 is controlled via the ground signals received from the antennas A. Further, train detection is performed based on the on-vehicle signals transmitted from the antennas A.
Since the principle of operation of the antenna A disposed on the train 1 for receiving ground signals is the same as the principle of operation of the antenna A for sending on-vehicle signals, only the operation of the antenna A for receiving ground signals will be described.
Both the antennas A disposed on the front side in the direction of travel of the train 1 and the antennas A disposed on the rear side in the direction of travel of the train 1 receive the magnetic field induced from the opposing loop coil 2 via the magnetic coupling. The four antennas A disposed on the train 1 have current components of the ground signals of the same level induced in the same direction. Since the antennas A disposed on the train 1 are series-connected to add the levels, the on-vehicle transceiver 6 receives transmission of the added ground signals received via the antennas A. When assuming that the level of ground signals received via the two antennas A disposed on the front side in the direction of travel of the train 1 is “1” and the level of ground signals received via the two antennas A disposed on the rear side in the direction of travel of the train 1 is “1”, the ground signals transmitted to the on-vehicle transceiver 6 will be “1”+“1”=“2”.
However, as shown in FIG. 9, when the train 1 stops so that the cross point 3 of the loop coil 2 is positioned between the two antennas A disposed on the front side in the direction of travel of the train 1 and the two antennas A disposed on the rear side in the direction of travel of the train 1, the current components of the ground signals flown through the loop coil 2 opposed to the two antennas A disposed on the front side in the direction of travel of the train 1 will be of reverse phase with the current components of the ground signals flown through the loop coil 2 opposed to the two antennas A disposed on the rear side in the direction of travel of the train 1. Since the direction of flow of the current components of the ground signals is reversed, the current component of the ground signals induced to the antennas A disposed on the front side in the direction of travel of the train 1 will be of reverse phase with the current component of the ground signals induced to the antennas A disposed on the rear side in the direction of travel of the train 1. Since the respective antennas A are series-connected to add the levels, the received ground signals will be of reverse phase and are cancelled out, so that no ground signals are transmitted to the on-vehicle transceiver 6. When assuming that the level of ground signals received via the two antennas A disposed on the front side in the direction of travel of the train 1 is “1”, the level of ground signals received via the two antennas A disposed on the rear side in the direction of travel of the train 1 will be of reverse phase from the ground signals received via the antennas A disposed on the front side, and will be “−1”. Since the respective antennas A are series-connected to add the levels, the ground signals transmitted to the on-vehicle transceiver 6 will be “1”+“−1”=“0”, according to which signals cannot be transmitted between the ground and the vehicle. When the ground signals applied to the loop coils 2 cannot be received, the train cannot continue operation.
Similarly, as shown in FIG. 10, when the train stops so that the boundary between two loop coils 2 to which ground signals of the same frequency and the same level are applied is positioned between the two antennas A disposed on the front side in the direction of travel of the train 1 and the two antennas A disposed on the rear side in the direction of travel of the train 1, the current component of the ground signals flown through the loop coil 2 opposed to the two antennas A disposed on the front side in the direction of travel of the train 1 will be of reverse phase with the direction of the current component of the ground signals flown through the loop coil 2 opposed to the two antennas A disposed on the rear side in the direction of travel of the train 1. Therefore, the current component of ground signals induced to the antennas A disposed on the front side in the direction of travel of the train 1 will be of reverse phase with the current component of the ground signals induced to the antennas A disposed on the rear side in the direction of travel of the train 1 and are cancelled out, so that no ground signals will be transmitted to the on-vehicle transceiver 6.
Japanese patent application laid-open publication No. 2001-199336 (patent document 1) discloses a known art for overcoming the aforementioned prior art problems, wherein when a train 1 stops above a cross point 3, the polarity of one antenna A out of the four antennas A disposed on the train is switched to achieve a given reception level, and when the traveling speed of the train 1 exceeds a predetermined speed, the switching is performed again so that the polarity of the four antennas become additive polarity.
According to a prior art on-vehicle transceiver device, when the train stops so that the cross point of the loop coil is positioned between antennas disposed on the front side in the direction of travel of the train and the antennas disposed on the rear side in the direction of travel of the train, the signals received via the antennas disposed on the front side in the direction of travel of the train will be of reverse phase with the signals received via the antennas disposed on the rear side in the direction of travel of the train and are cancelled out, so that it becomes impossible to transmit signals between the ground and the vehicle.
Further, when the train stops so that the boundary between two loop coils to which signals of the same frequency and the same level are applied is positioned between the antennas disposed on the front side in the direction of travel of the train and the antennas disposed on the rear side thereof, the signals received via the antennas disposed on the front side in the direction of travel of the train will be of reverse phase with the signals received via the antennas disposed on the rear side thereof and are cancelled out, so that it becomes impossible to transmit signals between the ground and the vehicle.
Not only an on-train transceiver device in which a plurality of antennas are disposed on the front and rear side in the direction of travel of the train but also an on-train transceiver for receiving ground signals via a single antenna has the aforementioned problems in that the signals cannot be transmitted between the ground and the train when the cross point of a loop coil or a boundary between two loop coils is positioned at the center of the antenna.
According further to the method disclosed in patent document 1, if the contact point for switching the polarity of antenna A malfunctions, it becomes impossible to transmit signals between the ground and the vehicle when the train stops above the cross point, according to which the train cannot continue operation.